The present invention relates to a method and an apparatus for controlling an injection molding machine. In particular, this invention relates to a method and an apparatus for controlling mold internal pressure in order to reduce variations in weight of molded products.
Referring to FIG. 1, a motor-driven injection molding machine will be described focusing on an injection unit. The motor-driven injection molding machine has the injection unit which is driven by a servomotor. In such an injection unit, rotation of the servomotor is converted into linear motion by a ball screw and a nut, thereby moving a screw forward and backward.
In FIG. 1, the rotation of an injection servomotor 11 is transmitted to a ball screw 12. A nut 13 is fixed on a pressure plate 14 and is moved forward and backward by rotation of the ball screw 12. The pressure plate 14 is movable along four guide bars 15 and 16 (only two are shown in the figure) fixed on a base frame (not shown). Forward and backward motion of the pressure plate 14 is transmitted to a screw 20 via a bearing 17, a load cell 18, and an injection shaft 19. The screw 20 is rotatably and axially movably disposed in a heating cylinder 21. The heating cylinder 21 includes a hopper 22 for feeding a resin to a position corresponding to the rear portion of the screw 20. Rotating motion of a servomotor 24 for rotating the screw 20 is transmitted to the injection shaft 19 via a connecting member 23 which may be a belt, pulleys, etc. In other words, the servomotor 24 rotates the injection shaft 19 which in turn rotates the screw 20.
In a plasticizing/measuring process, the screw 20 rotates and moves backward in the heating cylinder 21 so that a molten resin is stored in front of the screw 20, that is, in the heating cylinder 21 on the side of a nozzle 21-1. The backward movement of the screw 20 is caused by pressure due to gradual increase in the amount of molten resin stored in front of the screw 20.
In a filling and injecting process, the forward movement of the screw 20 in the heating cylinder 21 is caused by a driving force from the injection servomotor 11, so that the molten resin stored in front of the screw 20 is forced into and is pressurized in a metal mold. In this case, the force for pressing the molten resin is measured by the load cell 18 as an injection pressure. The measured injection pressure is amplified by a load cell amplifier 25 and is fed into a controller 26. The pressure plate 14 has a position detector 27 for measuring the amount of movement of the screw 20. The measuring signal outputted from the position detector 27 is amplified by a position detector amplifier 28 and is fed into the controller 26.
The controller 26 outputs current (torque) instruction values corresponding to the respective processes and based on some values preset by a display/setting unit 33 via a man-machine controller 34. The current instruction values are fed to a drive 29 and a drive 30. The drive 29 controls a current for driving the servomotor 11 to control an output torque of the servomotor 11. The drive 30 controls a current for driving the servomotor 24 to control the number of revolutions of the servomotor 24. The servomotor 11 and the servomotor 24 comprise encoders 31 and 32, respectively, for measuring the number of revolutions. The number of revolutions detected by the encoders 31 and 32 are fed to the controller 26. In particular, the number of revolutions detected by the encoder 32 is used to determine the number of revolutions of the screw 20.
If an injection molding machine is a hydraulic injection molding machine, a hydraulic injection cylinder may be used to replace the conversion system for converting a rotary movement into a linear movement, which is effected by means of the ball screw and the nut involved in the injection apparatus. In this case, the oil pressure within the injection cylinder is measured. Since the construction of the hydraulic injection molding machine is already well known in the art, it will not be illustrated and described in this specification.
However, one subject associated with the above-described injection molding machine is to manufacture, in a shortened time period and at reduced production cost, a large number of molded products of uniform quality. Here, one factor that has an influence on the quality of the molded products is their weights.
In order to manufacture molded products of uniform weight, there has been suggested a mold internal pressure feedback control system. In this mold internal pressure feedback control system, a mold internal pressure sensor is provided in the metal mold for measuring resin pressure in the metal mold. In this way, the injection servomotor or oil pressure in the hydraulic injection cylinder may be controlled in accordance with a difference between a mold internal pressure measured by the mold internal pressure sensor and a preset value of the mold internal pressure given as a target value, with the control itself being effected by reducing the difference to zero.
FIG. 2 is a block diagram showing the above-mentioned mold internal pressure feedback control system.
However, the mold internal pressure feedback control system shown in FIG. 2 has been found to have the following problems. That is, it is difficult to stabilize the pressure within the metal mold, merely by measuring the pressure variations within the metal mold since there are some limits to the response time of the overall control system. As a result, it is difficult to stabilize the mold internal pressure, hence making it difficult to manufacture molded products of uniform weight.
On the other hand, as discussed above, the mold internal pressure has a significant influence on the quality of the molded products, especially on the weights thereof. In view of this, there have been suggested some other control methods for stabilizing the mold internal pressure, which methods are different from that shown in FIG. 2. For example, a method has been suggested which requires that a mold internal pressure control system be connected in cascade with an injection pressure control system, a preset value of the mold internal pressure and a measured value of the same are compared with each other, so that a preset value of an injection pressure may be changed so as to adjust the injection pressure.
In the use of the above method, it is necessary that the injection pressure and the mold internal pressure be set at the same time when setting operation conditions. However, since the response of the mold internal pressure has an extremely large time lag in the injection pressure feedback system, it is not easy to adjust the injection pressure, nor is it easy to set the mold internal pressure, hence making it difficult to set operation conditions.
In fact, the mold internal pressure often changes in the manner shown in FIG. 3. In order to obtain molded products of a good quality, it is necessary that the mold internal pressure be set so that it can change in accordance with the graph shown in FIG. 3. This, however, makes it more difficult to set operation conditions.
Accordingly, it is an object of the present invention to provide a control method for controlling an injection molding machine, which is effective for producing molded products of a uniform weight.
Furthermore, another object of the present invention is to provide a control apparatus for controlling an injection molding machine, which is capable of stabilizing a mold internal pressure by an easy operation.
The control method of the present invention is suitable for use in an injection molding machine including a mold internal pressure sensor for measuring a mold internal resin pressure as a measured mold internal pressure value. The measured mold internal pressure value is fed to a nozzle internal resin pressure feedback control system. A nozzle internal pressure preset value in the nozzle internal resin pressure feedback control system is changed in accordance with a difference between the measured mold internal pressure value and an mold internal pressure preset value.
The control apparatus of the present invention is suitable for use in an injection molding machine which comprises a mold internal pressure sensor for measuring resin pressure within a metal mold cavity or within a resin flow passage extending to the cavity, a pressure sensor for detecting injection and dwell pressures, and injection and dwell pressure setting section, and a mold internal pressure setting section. The mold internal pressure setting section includes a memory. With respect to an injection and dwell pressure preset value fed from the injection and dwell pressure selling section, a detected value from the pressure sensor is fed back so as to set operation conditions. A pattern detected by the mold internal pressure sensor obtained when a molded product of a good quality was produced, is stored as the mold internal pressure present value in the memory. In an actual molding process, a detected value from the pressure sensor is fed back, and a difference is also fed back between the mold internal pressure preset value stored in the memory and a measured value fed from the mold internal pressure sensor, thereby adjusting an injection and dwell pressure preset value fed from the injection and dwell pressure setting section. In this way, an injection and dwell pressure feedback loop is used to perform a mold internal pressure feedback, thereby regulating an instruction value for injection and dwell.